Tp-Link TL-SG2424 [48/130] Q vlan

6.1 802.1Q VLAN

VLAN tags in the packets are necessary for the switch to identify packets of different VLANs. The

switch works at the data link layer in OSI model and it can identify the data link layer encapsulation

of the packet only, so you can add the VLAN tag field into the data link layer encapsulation for

identification.

In 1999, IEEE issues the IEEE 802.1Q protocol to standardize VLAN implementation, defining the

structure of VLAN-tagged packets. IEEE 802.1Q protocol defines that a 4-byte VLAN tag is

encapsulated after the destination MAC address and source MAC address to show the information

about VLAN.

As shown in the following figure, a VLAN tag contains four fields, including TPID (Tag Protocol

Identifier), Priority, CFI (Canonical Format Indicator), and VLAN ID.

Figure 6-2 Format of VLAN Tag

（1） TPID: TPID is a 16-bit field, indicating that this data frame is VLAN-tagged. By default, it is

0x8100 in this switch.

（2） Priority: Priority is a 3-bit field, referring to 802.1p priority. Refer to section “QoS & QoS

profile” for details.

（3） CFI: CFI is a 1-bit field, indicating whether the MAC address is encapsulated in the

standard format in different transmission media. This field is not described in detail in this

chapter.

（4） VLAN ID: VLAN ID is a 12-bit field, indicating the ID of the VLAN to which this packet

belongs. It is in the range of 0 to 4,095. Generally, 0 and 4,095 is not used, so the field is in

the range of 1 to 4,094.

VLAN ID identifies the VLAN to which a packet belongs. When the switch receives an

un-VLAN-tagged packet, it will encapsulate a VLAN tag with the default VLAN ID of the inbound

port for the packet, and the packet will be assigned to the default VLAN of the inbound port for

transmission.

In this User Guide, the tagged packet refers to the packet with VLAN tag whereas the untagged

packet refers to the packet without VLAN tag, and the priority-tagged packet refers to the packet

with VLAN tag whose VLAN ID is 0.

 Link Types of ports

When creating the 802.1Q VLAN, you should set the link type for the port according to its

connected device. The link types of port including the following three types:

（1） ACCESS: The ACCESS port can be added in a single VLAN, and the egress rule of the

port is UNTAG. The PVID is same as the current VLAN ID. If the ACCESS port is added to

another VLAN, it will be removed from the current VLAN automatically.

（2） TRUNK: The TRUNK port can be added in multiple VLANs, and the egress rule of the port

is TAG. The TRUNK port is generally used to connect the

cascaded network devices for it

can receive and forward the packets of multiple VLANs. When the packets are forwarded

by the TRUNK port, its VLAN tag will not be changed.